1. Field of the Invention
The present invention relates to a process for preparing or producing propylene resin sheets for thermoforming, which sheets are excellent in antifog property, transparency, rigidity and thermoforming property. Particularly, the present invention provides propylene resin sheets for thermoforming having a thermoforming property afforded thereto superior to the conventional ones and having antifog property, transparency and high rigidity in the production of transparent containers or lids used in the food industry.
2. Background Art
Sheet products made of plastic materials such as polystyrene or polyethylene terephthalate excellent in transparency are extensively used as containers or lid for containers because of their advantage that the content in the containers is easily identified. Recently, due to the increase in the use of frozen foods and foods to be heated in a microwave oven, containers or lid for containers made of sheets of such resins, however, have caused the problems of breakage or melting during the heating in a microwave oven, and thus transparent materials having antifog property, impact resistance, heat resistance as well as high rigidity have been requested by manufacturers of containers or lids.
On the other hand, propylene resin sheets excellent in heat resistance and impact resistance can be made transparent if materials and the processing used are adequately selected, but still have a problem that, when used as a lid material it may be difficult to identify therethrough the contents since its surface is hydrophobic and thus clouded with the water vapor from the contents.
There have been proposed such means for affording the antifog property to propylene resin sheets and the thermoformed articles made of the sheets as the use of propylene resin sheets having various antifogging agents blended in the resins or the use of propylene resin sheets having liquid antifogging agents applied thereon. However, propylene resin sheets and thermoformed articles made of the sheets such as containers or lids having satisfactory antifog property have not been presented yet as described below in more detail.
When containers are prepared from sheets of propylene resins having antifogging agents blended by the hot plate air-pressure forming, a propylene-ethylene random copolymer resin, for example as the propylene resin, can be formed into containers at a relatively wide range of forming temperatures, but there remain the problems that the rigidity of the containers formed would be lowered and/or the surface of the containers would be clouded due to the bleeding of the blended antifogging agent onto the surface of the sheet; that, when the containers are being thermoformed, the antifogging agent blended in the sheet would be transferred to the hot plate thereby clogging the pores (0.2 mm) for the application of vacuum or pressure in a mold to interrupt the continuous container forming operation; and that, when propylene resins having high rigidity and high crystallinity are used, pressure forming with a heating plate can be practiced only at a very narrow forming temperature range, whereby the antifogging agent blended is inhibited from bleeding at the crystalline part, so that the antifogging effect will not be exhibited.
In the method of coating containers with an antifogging agent, on the other hand, antifogging agents comprising sucrose fatty acid esters of lauric acid as their fatty acid residue have been used extensively (Japanese Patent Laid-Open Publication Nos. 166234/1981 and 80431/1982, and Japanese Patent Publication No. 36864/1986), but these antifogging agents have such problems that they have low heat resistant temperatures, and thus thermoformed articles such as containers or lids for containers which have been prepared by the thermoforming of propylene resin sheets having an antifogging agent applied thereon would not have antifog property endowed; and that if the amount of the antifogging agent is increased in order to improve antifog property, transparency would be lowered, the blocking of the coated sheet is frequently occurs, or the cost of the sheet is raised. There have been thus a demand for high-rigidity propylene resin sheets appropriate to the processing by the pressure forming with a hot plate and also for heat resistant antifogging coatings which are resistant to temperatures at the thermoforming.